Amphibious vehicle

ABSTRACT

A planing amphibious vehicle has at least one trim tab at its stern. A vehicle control system includes a mode change controller and a trim tab controller. The mode change controller informs the trim tab controller when a mode change event is taking place. The trim tab controller retracts the trim tabs if the mode change is from marine to land mode; and deploys the tabs if the change is from land mode to marine mode, to assist the vehicle in rising on to the plane. The controller may also retract the tabs if the vehicle reverses; and deploy the tabs if a change is made from reverse to forward motion. The vehicle control system may connect to actuators and sensors for retractable road wheels, which may use hydropneumatic struts. Safeguards against system faults and/or erroneous switch operation are included. Road wheel drive decouples are used; a marine drive decoupler may also be fitted.

BACKGROUND OF THE INVENTION

The present invention relates to an amphibious vehicle.

Amphibious vehicles are inherently required to carry both road vehicleand marine vessel dedicated equipment. Hence, amphibious vehicles aregenerally heavier than boats of a similar size and power. This isparticularly true in the case of planing amphibians, which require arelatively heavy wheel streamlining mechanism, such as retractablesuspension or the like. In the case of an amphibious vehicle with openwheel arches, the drag exerted by the arches can be considerable andhence the overall hull drag is greater than for a conventional boat.This additional weight and drag make it difficult for the vehicle torise onto the plane without initial deployment of trim tabs.

Trim tabs are well known for marine vessels, and in particular planingboats, as means for controlling the trim and attitude of the vessel tocompensate for changes in load, speed or sea conditions. Commonly thesedevices are in the form of flat tabs or planes, which are pivotablyconnected along a generally horizontal axis to the hull of the vesselbelow the waterline near the stem. The angle of orientation of the trimtabs is adjustable and hence determines the fore and aft attitude of thevessel, when it is being propelled through the water. For example, ifthe rear of the vessel is heavily loaded the bow will typically rise outof the water and lead to inefficient forward motion. In thesecircumstances the angle of the trim tabs relative to the hull can beincreased to lower the bow, lift the stem, and hence get the boat backonto a plane.

The position of the trim tabs on a conventional boat, when docked, isnot of great importance since it is unlikely that the tabs will comeinto contact with, and be damaged by a submerged obstacle. Nevertheless,since trim tabs are typically extended by means of hydraulic actuatorsit is generally desirable for trim tabs to be retracted, when the boatis not in use, to avoid unnecessary marine growth on the actuators.Retraction also minimises the risk of damage when a boat is lifted fromthe water, for example, to go into dry storage. In accordance with thisU.S. Pat. No. 5,113,780 discloses a trim tab control system, whichincludes a facility for automatically retracting the trim tabs when theengine ignition is switched off.

The system described in U.S. Pat. No. 5,113,780 is designed specificallyfor marine only vessels and is of limited benefit in the case ofamphibious vehicles fitted with trim tabs. The greatest risk of damage,in the case of amphibious vehicles, occurs during and after thetransition from marine to terrestrial mode, whilst the engine is stillswitched on. If the tabs remain in an operational position when thevehicle leaves the water, there is a significant risk that they maystrike the ground at speed. Similarly, reversing into an obstacle withthe trim tabs deployed could damage the tabs, the actuators and the bodyof the vehicle. The risk of inadvertently leaving the trim tabs deployedis enhanced because during the marine to terrestrial transition periodthe operator has several other important control tasks to consider.

Furthermore, the requirement in U.S. Pat. No. 5,113,780 that the tabs beretracted after the ignition is switched off necessitates an ignitionindependent power supply for powering the retraction system. Such asupply, however, is undesirable because the power source remainspermanently connected to the circuit that it powers. Hence, even arelatively small current drain from the retraction circuit could lead toa significant reduction in the life of the source. Whilst a permanentcurrent drain may be unlikely to arise as a result of poor design thereis a significant probability that it could occur as a result ofmalfunction. The risk of this is particularly high in marine vesselswhere the presence of water makes short circuits more likely. It willalso be appreciated that short circuits of this type also constitute asafety hazard.

This latter problem has been tackled in U.S. Pat. No. 5,474,013, whichdescribes a system for automatically retracting trim tabs on a boatindependently of the operation of the ignition switch. In the systemdescribed, a capacitor is charged from the vessels battery and thecharge stored can be discharged to fully retract the trim tabs. However,this system is also designed specifically for marine only vessels andagain is of limited benefit in the case of amphibious vehicles, for thereasons discussed above in relation to U.S. Pat. No. 5,113,780.

There is, therefore, a need for an amphibious vehicle, which includes atrim tab deployment system to allow it to rise onto the plane. There isalso a need for the trim tab system for the amphibious vehicle toovercome or at least to mitigate the problems referred to above.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided anamphibious vehicle adapted for use on land and on water including avehicle body, and at least one trim tab, for adjusting the trim of theamphibious vehicle when it is in a marine mode, connected to the vehiclebody and movable relative to the vehicle body between a retractedposition and any one of a range of operational positions, and a controlsystem for controlling the position of the or each trim tab includingdetecting means for detecting at least one mode change event and tabactuation means for moving the or each trim tab relative to the vehiclebody, the control system being adapted to automatically move the or eachtrim tab either into the retracted position or into any one of a rangeof operational positions according to the mode change event detected.

Preferably, the mode change event, detectable by the detecting means, isa first mode change event indicative of an onset of transition into amarine mode from a terrestrial mode and the control system is adapted toautomatically move the or each trim tab into one of a: range ofoperational positions on detection of said first mode change event.

Preferably the or a further mode change event, detectable by thedetecting means, is a second mode change event indicative of an onset ofa transition into the terrestrial mode from a marine mode and thecontrol system is adapted to automatically move the or each trim tabinto the retracted position on detection of said second mode changeevent.

Preferably the detecting means are also capable of detecting selectionof reverse propulsion in a marine mode, and the control system isadapted to automatically move the or each trim tab into the retractedposition on detection of the selection of reverse propulsion.

Preferably the detecting means are also capable of detecting a selectionof forward propulsion when the vehicle is moving in the reversedirection in a marine mode and the control system is adapted toautomatically move the or each trim tab into any one of a range ofoperational positions on the selection of forward propulsion.

Preferably the control system includes means for signalling an operatoron detection of at least one event.

Preferably the vehicle has a set of wheels for supporting the vehiclewhen it is in terrestrial mode, and the control system further includesmeans for retracting the set of wheels on detection of a transition intomarine mode and means for deploying the set of wheels on detection of atransition into terrestrial mode.

Preferably the vehicle has a jet drive for propelling the vehicle whenit is in marine mode and a reversing bucket, and the control systemincludes means for deploying the reversing bucket on detection ofselection of reverse propulsion. Alternatively, where a reversing bucketis not fitted, the jet drive may be driven in reverse through thevehicle transmission.

According to another aspect of the present invention there is provided amethod for controlling a trim tab system for an amphibious vehicleincluding the steps of automatically detecting a mode change event, andon detection of the mode change event automatically moving the or eachtrim tab either into the retracted position or into any one of a rangeof operational positions according to the event detected.

Preferably the mode change event detected is an onset of transition intoa terrestrial mode.

Preferably the mode change event detected is an onset of transition intoa marine mode.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the invention will now be described, by way of exampleonly, with reference to the following drawings in which:

FIG. 1 shows diagrammatically a simplified side view, partially cutaway, of an amphibious vehicle according to the invention;

FIG. 2 shows diagrammatically a simplified plan view, from beneath, ofthe amphibious vehicle of FIG. 1;

FIG. 3 is a functional block diagram of part of an integrated controlnetwork for the amphibious vehicle of FIG. 1;

FIG. 4 shows a control system logic sequence to initiate a mode change;

FIG. 5 shows a control system logic sequence for a mode change fromterrestrial to marine mode, as initiated by the sequence shown in FIG.4; and

FIG. 6 shows a control system logic sequence for a mode change frommarine to terrestrial mode, as initiated by the sequence shown in FIG.4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIGS. 1 and 2 an amphibious vehicle is designated generally 10 and atrim tab system is designated generally 12. The amphibious vehicle 10 iscapable of operation in both a terrestrial mode, for travel on land, anda marine mode, for travel on water, The vehicle 10 includes a vehiclebody 14 having a hull section 16 and an outer body section 18. The hullsection 16 is equivalent to the hull of a conventional boat and includesa transom 20 at the stern.

The trim tab system 12 includes two tabs 22 and two hydraulic actuators26, provided one on either side of a central longitudinal axis AA′ ofthe vehicle 10 as seen in FIG. 2. Whilst the figures show hydraulicactuators, any suitable actuation means may be used, for exampleelectrically powered actuators. Each of the tabs 22 is connected to thebase of the transom 20 by a corresponding hinge 24 along a generallyhorizontal axis to allow independent rotational movement of each tabrelative to the transom 20, as seen on FIG. 1.

The hydraulic actuators 26 are of conventional construction, eachactuator 26 having a cylinder 28 into which one end of an actuator rod30 is slidably received. The cylinder 28, of each hydraulic actuator 26,is mounted pivotally on the transom 20 above a corresponding tab 22 towhich the protruding end of the actuator rod 30 is pivotally connected.Hence, in operation, extension and retraction of the actuator rod 30lowers and raises the corresponding tab 22 respectively.

The amphibious vehicle 10 has the standard features associated with aroad vehicle including transmission, suspension and a set of wheels 32.The wheels 32 are provided with a retraction mechanism, which can beused to retract the wheels 32 from a protracted position 31 during atransition into marine mode. Similarly, the retraction mechanism can beused to protract the wheels 32 from a retracted position 33 during atransition into terrestrial mode. As shown in FIG. 3, the retractionmechanism makes up part of a mode change system 54 for converting theamphibious vehicle 10 from marine mode to terrestrial mode and viceversa.

The retraction mechanism may comprise any suitable means for retractingand protracting the wheels 32. Preferably, however, the retractionmechanism is of the form described in European Patent No. 0742761. Theretraction mechanism, as such, does not form part of the presentinvention and so will not be described here in detail. However, if thereader requires details of the operation and construction of a suitableretraction system then they should refer to EP 0742761, the contents ofwhich are hereby incorporated by reference.

The suspension may be a conventional hydro-pneumatic system withsuspension cylinders part filled with hydraulic fluid and part filledwith pneumatic gas. Prior to road use, the suspension cylinders of theamphibious vehicle require charging to bring the vehicle to the correctroad height. Typically, the charging will occur either when the vehicleengine is switched on in terrestrial mode or when the vehicle makes atransition from marine into terrestrial mode. It should be understood,however, that the vehicle may have any suitable suspension system.

Additionally, the amphibious vehicle 10 includes a marine drive 34 forproviding propulsion when the vehicle 10 is in the marine mode. Themarine drive 34 is a conventional jet drive, which in operation issues ajet of high pressure water to propel the vehicle 10 forward. The jetdrive may be fitted with a reversing bucket 36, which in use, may bedeployed to divert the direction of the water jet to propel the vehicle10 in reverse. Alternatively, where a reversing bucket is not fitted,the direction of rotation of the jet drive may be reversed through thevehicle transmission. It will be appreciated that other forms of marinedrive could be employed, with corresponding reversing mechanisms, suchas a marine propeller for example.

The mode change system 54 also includes means for decoupling the roadwheel transmission and optionally engaging the marine drive 34 duringtransition into marine mode and means for charging the suspension,coupling the road wheel transmission and optionally disengaging themarine drive 34 during transition into terrestrial mode.

Referring now to FIG. 3, the trim tab system 12 also includes anelectro-hydraulic system 40 for independently extending and retractingthe rods 30 and hence operating the tabs 22. The electro-hydraulicsystem 40 includes an electric motor 42, which in operation, drives afluid pump 44 to move fluid under pressure, via flow control valves 46,either to or from the actuators 26, to extend or retract the actuatorrods 30 respectively. The flow control valves 46 allow independentselection and control of the actuators 26. Alternatively, the system forindependently extending and retracting the rods 30 could be purelyelectromechanical employing electric motors to operate mechanicalactuators with no hydraulic parts.

In order to control the electro-hydraulic and electromechanical systemsan integrated control network 50 is provided. The control network 50 hasa main controller 52, a set of input subsystems 56, and a set of outputsubsystems 58. The main controller 52 comprises a set of controlsubsystems 60, 62, 64, which include a propulsion controller 60, a modechange controller 62 and a trim tab controller 64. The main controller52 can be implemented using any suitable means such as a dedicated logiccircuit or a pre-programmed micro-controller.

The input subsystems 56 include manually operated switches such as areverse selector 66, a mode change switch 68 and manual trim tab controlswitches 70. The input subsystems 56 also include sensor meanscomprising suspension droop sensor apparatus 74, for determining if thewater buoyantly supports the vehicle 10. The sensor system may compriseany suitable means for detecting when the vehicle 10 is in water andbuoyantly supported by it. An amphibious vehicle incorporating suitablesensor means is described in the applicant's International patentapplication number PCT/GB2002/005359 that claims the benefit of priorityfrom British application No. 0128338.1, the contents of which are herebyincorporated by reference. A water presence sensor 72 may also beprovided, as a secondary sensor.

The output subsystems 58 include the mode change system 54, the trim tabsystem 12, a reversal system 76 and warning systems 48, 78.

In operation, the main controller 52 accepts external inputs from theinput subsystems 56 and directs them to at least one of the controlsubsystems 60, 62, 64 for processing. The control subsystems 60, 62, 64,process the inputs and produce a corresponding set of outputs, whichdetermine, and hence control, the behaviour of the output subsystems 58.The outputs from any control subsystem 60, 62, 64, can also forminternal inputs to any other control subsystem 60, 62, 64.

As seen in FIG. 3 the trim tab controller 64 receives external inputsfrom the trim tab control switches 70 and the reverse selector 66, andan internal input from the mode change controller 62. The trim tabcontroller 64 produces a corresponding set of outputs for controllingthe trim tab system. More specifically the trim tab controller 64controls the motor 42, the pump 44, the flow control valves 46 and hencethe relative positioning of the tabs 22. The trim tab controller 64 alsocontrols a trim tab warning system 48 for alerting the vehicle operator,when appropriate, to the position of the trim tabs 22.

The reverse selector 66 also provides an external input to thepropulsion controller 60, which in turn produces an output to thereversal system 76 for either deploying or retracting the reversingbucket 36; or alternatively to reverse the rotation direction of thevehicle transmission.

The mode change controller 62 receives external inputs from the modechange switch 68, the suspension droop sensor 74 and the water presencesensor 72. The mode change controller 62 processes these external inputsand produces outputs to the mode change system 54, a mode change warningsystem 78, the propulsion controller 60 and trim tab controller 64.

The mode change warning system 78 includes a visual warning device inthe form of a warning light to alert the operator when the vehicle isundergoing a mode change and to inform the operator when the transitionis complete. The mode change warning system 78 also includes an audiblewarning device in the form of a buzzer for warning people in thevicinity of the vehicle 10 when a mode change is imminent.

It will be appreciated that the some or all of the systems andsubsystems making up the control network 50 maybe equipped with resetfunctions (not shown) for resetting the whole system or individualsystems if required, for example, in the event of problems during modechanges.

In operation, the control system 50 follows the logic sequences shown inFIGS. 4 and 5 to affect a transition from terrestrial into marine modeand the logic sequences shown in FIGS. 4 and 6 to affect a transitionfrom marine into terrestrial mode. The logic sequences include severalmode change events interspersed with decision points. Mode change eventsare events that are associated with transition either to terrestrialmode or to marine mode.

In FIG. 4 a control system logic sequence to initiate a mode change isdesignated generally 90. While the vehicle 10 is operational, the modechange controller 62 monitors the status of the mode change switch 68 todetermine if it has been activated. On activation of the mode changeswitch 68, the logic sequence 90 initially establishes whether theswitch has been activated in error. If the switch 68 is activated forless than three seconds then this is deemed to be indicative ofaccidental operation, for example by an operator pressing the wrongswitch. Hence, no further action is taken and the mode change controller62 reverts to its monitoring activity.

If the switch 68 is activated for more than three seconds then the modechange controller goes on to establish if the switch 68 is releasedwithin ten seconds. If the switch 68 remains activated after ten secondsthen this is deemed to be indicative of malfunction or an enduringaccidental operation, for example by an object being placed on oragainst the switch 68. Hence, no further action is taken and the modechange controller 62 reverts to its monitoring activity. An alert systemcould also be included which warns the operator that the switch 68 haseither been erroneously operated or is malfunctioning.

Thus, if the duration of the switch activation is between three and tenseconds the mode change controller 62 signals the mode change warningsystem 78 to flash the visual warning device and sound the audiblewarning device, prior to determining the current status of the vehicleand initiating an appropriate mode change logic sequence.

In FIG. 5 a control system logic sequence for a mode change fromterrestrial to marine mode is designated generally 100. On initiation,the mode change controller 62 monitors the external inputs from thesensor subsystem to determine if conditions are suitable for transitionto occur. If the external input to the controller 62, from thesuspension droop sensor apparatus 74 indicates that the vehicle 10 isnot buoyantly supported, sufficient to allow safe retraction of thewheels 32, no action is taken and monitoring continues. However, if thesuspension droop sensor apparatus 74 indicates that safe retraction ofthe wheels 32 is possible, the mode change controller 62 goes on to testfor external water presence by monitoring the external input receivedfrom the water presence sensor 72. When external water presence isconfirmed the vehicle 10 is deemed to be in water. If full droop and/orwater presence are not detected within thirty seconds then conversion isstopped and the control system reverts to its general monitoring mode.

At this stage a system check is carried out and any faults detectedresult in termination of the process. If no faults are detected then thetransition to marine mode can begin.

Transition to marine mode involves the mode change controller 62initiating a number of mode change events 108, 110, 112, 114. Inaccordance with this, outputs are produced for controlling the modechange system 54, the mode change warning system 78, and for furtherprocessing by the trim tab controller 64 and the propulsion controller60.

The mode change controller 62 signals the mode change system 54 toretract the wheels 32 and to decouple the road wheel transmission of thevehicle 10. Similarly, where a marine drive decoupler is fitted, thepropulsion controller 60 receives an internal input from the mode changecontroller 62 and makes a subsequent output to the mode change system 54to engage the marine drive 34. After this has occurred the mode changecontroller 62 signals the mode change warning system 78 to switch thevisual warning device from flashing to continuous and the audiblewarning device to silent.

Additionally, on receipt of the internal input from the mode changecontroller 62 the trim tab controller 64 signals the trim tab system toautomatically deploy the trim tabs from a retracted position 80 into amarine ready position 82. As seen on FIG. 1 the retracted position 80lies at an angle substantially 13° above the horizontal and the marineready position 82 lies substantially at an angle between 9° and 12°below the horizontal.

At this stage the mode change controller 62 interacts with the modechange system to establish when the wheels 32 are retracted. Once thewheels 32 are retracted the vehicle is ready for planing. However, fillengine power can be applied as soon as the road wheel drive isdisengaged.

Whilst FIG. 5 shows the mode change events 108, 110, 112, 114 occurringsequentially it will be appreciated that there are other sequences thatmaybe followed and some or all of the events 108, 110, 112, 114 may runconcurrently to speed up the transition.

In FIG. 6 a control system logic sequence for a mode change from marineto terrestrial mode is designated generally 120. Initially a systemcheck is carried out and any faults detected result in termination ofthe process. If no faults are detected then the transition toterrestrial mode can begin.

As with the transition to marine mode, the transition to terrestrialmode involves the mode change controller 62 initiating a number of modechange events 124, 128, 130, 132, 134. In accordance with this, outputsare produced for controlling the mode change system 54, the mode changewarning system 78, and for further processing by the trim tab controller64 and the propulsion controller 60.

The mode change controller 62 signals the mode change system 54 toprotract the wheels 32 and then suspends further commands until thesuspension droop sensor 74 indicates that the suspension has reachedfull travel. The mode change controller 62 then signals the mode changesystem 54 to charge the suspension and couple the transmission ready forroad use. Similarly, where a marine drive decoupler is fitted, thepropulsion controller 60 receives an internal input from the mode changecontroller 62 and makes a subsequent output to the mode change system 54to disengage the marine drive 34.

Additionally, on receipt of an internal input from the mode changecontroller 62 the trim tab controller 64 signals the trim tab system toautomatically move the trim tabs to the retracted position 80 from anyone of a range of operational positions. After retraction has occurredthe mode change controller 62 signals the mode change warning system 78to switch the visual warning device from flashing to off and the audiblewarning device to silent.

Whilst FIG. 6 shows the mode change events 124, 128, 130, 132, 134occurring sequentially it will be appreciated that there are othersequences that may be followed and some or all of the events 124, 128,130, 132, 134 may run concurrently to speed up the transition.

When the vehicle 10 is operating in the marine mode the operator can usethe trim tab control switches 70 to manually adjust the position of thetrim tabs 22 depending on external conditions, the forward speed of thevehicle 10, vehicle loading and any other requirements. In use, the trimtab control switches 70 provide an external input to the trim tabcontroller 64. The trim tab controller 64 processes this external inputand makes a resulting output to the trim tab system 12 to eitherincrease or decrease the angle of the tabs 22. For example, when thevehicle begins motion under forward propulsion the operator willtypically increase the angle of the tabs from the marine ready positionto an operational position. As the vehicle 10 accelerates and the bowbegins to rise, the operator can further increase the angle of the tabsto correct fore and aft attitude. Similarly, if the vehicle 10 isunevenly loaded, such that it lists to one side, the tabs 22 maybeoperated independently to correct the port/ starboard attitude of thevehicle 10 when it is in motion.

It will be appreciated that much of the manual tab control can befurther automated to take account of standard conditions and loading.For example, the trim tab controller 64 could be provided with internalinputs from the propulsion controller 60 to automatically adjust thetabs depending on speed. Alternatively, a separate speed sensor could beused. A feedback mechanism involving a gyroscope, yaw rate sensor, orother attitude detector could also be included, which would allowautomatic adjustment of the tabs depending on the attitude of thevehicle. Tab position feedback, to controller 64, would be required forfully automatic trim tab control.

When the vehicle 10 is operating in the marine mode, under forwardpropulsion, and reverse propulsion is then selected, the reverseselector 66 provides external inputs, corresponding to reverseselection, to the trim tab and propulsion controllers 64, 60.Consequently, the external inputs are processed by the appropriatecontrol subsystem 60, 64 and appropriate outputs made to the relatedoutput subsystems 12, 76. Specifically, the trim tab controller 64 makesoutputs to the trim tab system 12 to move the tabs 22 to the retractedposition 80, and the propulsion controller 60 makes an output to thereversal system 76 to initiate the deployment of the reversing bucket36.

Conversely, when the vehicle 10 is operating in the marine mode, underreverse propulsion, and forward propulsion is then selected, the reverseselector 66 provides external inputs, corresponding to forwardselection, to the trim tab and propulsion controllers 64, 60. In thiscase, after processing the external input, the propulsion controller 60initiates the retraction of the reversing bucket 36, where fitted, bythe reversal system 76. Correspondingly, the trim tab controller 64initiates the deployment of the tabs 22, to the marine ready position82, by the trim tab system 12.

It will be appreciated that the control system 50 may also be configuredto respond to other events, such as engine ignition being switched on oroff. For example, when the vehicle 10 is in the marine mode with theengine off, and the ignition is then switched on, external inputs maybeprovided to the trim tab controller 64 to move the tabs 22 to the marineready position 82. Similarly, when the engine is switched off externalinputs may be provided to the trim tab controller 64 to fully retractthe tabs 22 or to move them to any other suitable default position.

Whereas the invention has been described in relation to what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not limited to thedisclosed arrangements but rather is intended to cover variousmodifications and equivalent constructions included within the spiritand scope of the invention.

1. An amphibious vehicle adapted for use on land and on water including: a vehicle body; at least one trim tab, for adjusting the trim of the amphibious vehicle when it is in a marine mode, the at least one trim tab connected to the vehicle body and movable relative to the vehicle body between a retracted position and any one of a range of operational positions; and a control system for controlling the position of the at least one trim tab, the control system including detecting means for detecting at least one mode change event and tab actuation means for moving the at least one trim tab relative to the vehicle body, the control system being adapted to automatically move the at least one trim tab either into the retracted position or into any one of a range of operational positions according to the mode change event detected, wherein the mode change event, detectable by the detecting means, is a first mode change event indicative of an onset of transition into a marine mode from a terrestrial mode and the control system is adapted to move the at least one tab into one of a range of operational positions on detection of the said first mode change event wherein the detecting means are capable of detecting selection of reverse propulsion in a marine mode, and the control system is adapted to automatically move the at least one trim tab into the retracted position on detection of the selection of reverse propulsion; and wherein the detecting means are capable of detecting a selection of forward propulsion when the vehicle is moving in the reverse direction in a marine mode, and the control system is adapted to automatically move the at least one trim tab into any one of a range of operational positions on the selection of forward propulsion.
 2. An amphibious vehicle having a transom section, adapted for use on land and on water including: a vehicle body; at least one trim tab, for adjusting the trim of the amphibious vehicle when it is in a marine mode, the at least one trim tab connected to the vehicle body at a location proximate the transom section, wherein each trim tab is independently movable relative to the vehicle body between a retracted position and any one of a range of operational positions; and a control system for controlling the position of the at least one trim tab including detecting means for detecting at least one mode change event and tab actuation means for moving the at least one trim tab relative to the vehicle body, the control system being adapted to automatically move the at least one trim tab either into the retracted position or into any one of a range of operational positions according to the mode change event detected wherein the control system includes means for signaling an operator on detection of at least one event, wherein the mode change event, detectable by the detecting means, is a first mode change event indicative of an onset of transition into a marine mode from a terrestrial mode and the control system is adapted to move the at least one tab into one of a range of operational positions on detection of the said first mode change event.
 3. An amphibious vehicle according to claim 2, in which one of the first mode change event and a further mode change event, detectable by the detecting means, is a second mode change event indicative of an onset of transition into the terrestrial mode from a marine mode and the control system is adapted to automatically move the at least one trim tab into the retracted position on detection of the said second mode change event.
 4. A method for controlling the trim tab system of the amphibious vehicle according to claim 3 including the steps of automatically detecting the mode change event, and on detection of the mode change event automatically moving the at least one trim tab either into the retracted position or into said any one of the range of operational positions according to the event detected wherein the mode change event detected is an onset of transition into the terrestrial mode from the marine mode.
 5. A method for controlling the trim tab system of the amphibious vehicle according to claim 2 including the steps of automatically detecting the mode change event, and on detection of the mode change event automatically moving the at least one trim tab either into the retracted position or into said any one of the range of operational positions according to the event detected wherein the mode change event detected is an onset of transition into the marine mode from the terrestrial mode.
 6. An amphibious vehicle, adapted for use on land and on water including: a vehicle body; at least one trim tab, for adjusting the trim of the amphibious vehicle when it is in a marine mode, connected to the vehicle body and movable relative to the vehicle body between a retracted position and any one of a range of operational positions; and a control system for controlling the position of the at least one trim tab including detecting means for detecting at least one mode change event and tab actuation means for moving the at least one trim tab relative to the vehicle body, the control system being adapted to automatically move the at least one trim tab either into the retracted position or into any one of a range of operational positions according to the mode change event detected in which the vehicle has a set of wheels for supporting the vehicle when it is in terrestrial mode, and the control system includes means for retracting the set of wheels on detection of a transition into marine mode and means for deploying the set of wheels on detection of a transition into terrestrial mode.
 7. An amphibious vehicle, adapted for use on land and on water including: a vehicle body; at least one trim tab, for adjusting the trim of the amphibious vehicle when it is in a marine mode, connected to the vehicle body and movable relative to the vehicle body between a retracted position and any one of a range of operational positions; and a control system for controlling the position of the at least one trim tab including detecting means for detecting at least one mode change event and tab actuation means for moving the at least one trim tab relative to the vehicle body, the control system being adapted to automatically move the at least one trim tab either into the retracted position or into any one of a range of operational positions according to the mode change event detected wherein the vehicle has a jet drive for propelling the vehicle when it is in marine mode and a reversing bucket, and the control system includes means for deploying the reversing bucket on detection of selection of reverse propulsion.
 8. An amphibious vehicle adapted for use on land and on water including: a vehicle body; at least one trim tab, for adjusting the trim of the amphibious vehicle when it is in a marine mode, connected to the vehicle body and movable relative to the vehicle body between a retracted position and any one of a range of operational positions; and a control system for controlling the position of the at least one trim tab including detecting means for detecting at least one mode change event and tab actuation means for moving the at least one trim tab relative to the vehicle body, the control system being adapted to automatically move the at least one trim tab either into the retracted position or into any one of a range of operational positions according to the mode change event detected; wherein the mode change event, detectable by the detecting means, is a first mode change indicative of an onset of transition into a marine mode from a terrestrial mode and the control system is adapted to move the at least one tab into one of a range of operational positions on detection of the said first mode change event; wherein the detecting means are capable of detecting selection of reverse propulsion in a marine mode, and the control system is adapted to automatically move the at least one trim tab into the retracted position on detection of the selection of reverse propulsion; and wherein the detecting means are capable of detecting a selection of forward propulsion when the vehicle is moving in the reverse direction in a marine mode, and the control system is adapted to automatically move the at least one trim tab into any of the range of operational positions on the selection of forward propulsion.
 9. An amphibious vehicle adapted for use on land and on water including; a vehicle body; at least one trim tab, for adjusting the trim of the amphibious vehicle when it is in a marine mode, connected to the vehicle body and movable relative to the vehicle body between a retracted position and any one of a range of operational positions; and a control system for controlling the position of the at least one trim tab including detecting means for detecting at least one mode change event and tab actuation means for moving the at least one trim tab relative to the vehicle body, the control system being adapted to automatically move the at least one trim tab either into the retracted position or into any one of a range of operational positions according to the mode change event detected, wherein the vehicle has a jet drive for propelling the vehicle when it is in marine mode and a reversing bucket, and the control system includes means for deploying the reversing bucket on detection of selection of reverse propulsion. 